1. Field of the Invention
The present invention relates to a machine tool thermal change computation apparatus for use with a machine tool to compute a thermal change amount generated in the machine tool and a storage medium for storing a program for such a thermal change computation.
2. Description of Related Art
Machine tools include generally a machining mechanism for cutting or drilling a workpiece and assembling product component parts on a base plate, and a driving mechanism for changing a relative position between the machining mechanism and a workpiece. In general, machine tools for cutting a workpiece or the like include a holding mechanism for holding tools such as drills and taps, a main driving mechanism for rotating tools held on the holding mechanism, an X-axis feed mechanism for feeding tools in the X-axis direction, a Y-axis feed mechanism for feeding tools in the Y-axis direction, a Z-axis feed mechanism for feeding tools in the Z-axis direction, an electronic control apparatus for controlling these feed mechanisms and the like.
For example, as shown in FIG. 18, a machine tool 10 includes a splash guard 12 for preventing cut wastes from splashing. In the inside of the splash guard 12, there are disposed a table 14 for holding a workpiece (not shown), an automatic tool change (ATC) magazine 16 for changing tools such as drills and taps, a machine tool body 20 and the like. The splash guard 12 includes a control panel 22, a work change opening 24 for loading or ejecting a workpiece and an inspection hatch 26 used mainly for maintenance and the like.
As shown in FIG. 19, the body 20 includes a main shaft 28 for holding tools such as drills and taps, a main shaft motor 30 for rotating the main shaft 28, a ball thread mechanism 36 which comprises a nut unit 32 housing a number of steel balls and which is fixed to the main shaft side and a ball thread 34 inserted into the nut unit 32, a Z-axis motor 38 for rotating the ball thread 34, a guide rail 40 disposed in parallel with the ball thread 34, a slide 42 for coupling the guide rail 40 and the main shaft 28 or the like.
In this body 20, the ball thread mechanism 36 and the Z-axis motor 38 constitute the Z-axis feed mechanism for feeding the main shaft 28 in the Z-axis direction (up-down direction in FIG. 15). When the Z-axis motor 38 rotates the ball thread 34, the main shaft 28 is moved in the Z-axis direction. Moreover, the table 14 shown in FIG. 14 may be moved in the X-axis direction and Y-axis direction. As the main shaft 28 may be moved in the Z-axis direction, relative positions among a workpiece and tools in the X-axis, Y-axis and Z-axis directions may be changed.
In this type of machine tool, as the ball thread mechanism 36 is operated, a frictional heat is generated and the ball thread 34 expands. Also in other mechanisms, a thermal change occurs due to the heat thus generated. When such thermal change occurs in the Z-axis direction, for example, an error occurs in a depth of flute and a height of stepped portion formed on a workpiece. If a tolerance is sufficiently larger than a thermal change amount, then a machining error caused by such thermal change is negligible. If not, such a thermal change should be corrected.
Therefore, there is provided a thermal change amount computation apparatus for computing a thermal change amount of a machine tool. In this computation apparatus, when a drive mechanism is controlled in accordance with a previously-determined work program, the drive mechanism is controlled while a machining error is corrected in response to the computed thermal change amount (JP-A-62-88548).
However, in this machine tool thermal change amount computation apparatus, since the thermal change amount is continuously computed during the machine tool is operated, a system for executing such computation processing should be operated constantly as well, resulting in an increase in the computation processing load.
In view of the fact that a heat generated amount and a heat radiated amount are placed in the equilibrium state as the temperature rises during a machine tool is continuously operated, a method of computing a thermal change amount is proposed in JP application No. 8-298866 by the same assignee as the present application. That is, during a period in which the machine tool is operated, a thermal change amount at each timing point is computed based on a saturated thermal change amount (thermal change amount in the equilibrium state) and a machine tool driving time. When the computed thermal change amount becomes nearly equal to the saturated thermal change amount, the value of the saturated thermal change amount will be used as the thermal change amount. In this case, if an accurate saturated thermal change amount is given, a thermal change amount at each timing point may be computed thus reducing a computation processing load.
However, in the above machine tool thermal change amount computation apparatus, a thermal change amount computation accuracy is lowered a little in the case where a machine tool operating program includes a step for driving a machine tool at a high speed and step for driving a machine tool at a low speed. If the saturated thermal change amount is fixed relative to an average driving speed, it is unavoidable that the above computation method computes a thermal change amount as a smaller thermal change amount in step of driving a machine tool at a high speed and computes a thermal change amount as a larger thermal change amount in step of driving a machine tool at a low speed.